1. Field of the Invention
The present invention relates to a bandwidth allocation method, an optical line terminator, an optical network unit, a communication system, and a recording medium recording a program of a device in a system, such as Passive Optical Network (hereinafter “PON”), in which the optical line terminator and the optical network unit are connected.
2. Description of Related Art
In recent years, the broadband of access lines has progressed as a result of the rapid spread of the Internet. Examples of broadband access lines in practical use include various systems such as ADSL (Asymmetric Digital Subscriber Line) and cable modems. The PON is considered a strong potential worldwide for wider bandwidth.
FIGS. 1 and 2 depict a general configuration of the PON. ONUs (optical network unit) are installed in end users' houses, and an OLT (optical line terminator) is installed at a station. The ONUs and OLT are connected by optical fibers and an optical splitter. The personal computers of the users are connected to the network through the ONUs and further connected to superior networks and the Internet through the OLT.
As uplink signals (wavelength is usually 1.3 μm) and downlink signals (wavelength is usually 1.5 μm) are wavelength-multiplexed, the devices are connected by an interactive single-core optical fibers. The downlink signals are broadcasted from the OLT to all ONUs, and each ONU checks the address of the frame and imports the frame addressed to the ONU.
The uplink signals from the ONUs merge at the optical splitter, and time-division multiplexing is used to avoid collision of the signals. Therefore, the OLT adjusts output requests (REPORT) momentarily reported from the ONUs and provides signal transmission permissions (GATE) to the ONUs after calculating the transmission time based on the distances between the OLT and the ONUs.
The output request (REPORT) includes information of queue status (length of queue) of buffers. The signal transmission permission (GATE) includes transmission start time and transmission duration time for each priority of signals, and the ONUs transmit the uplink signals according to the times. Thus, the uplink bandwidth allocation is realized by allocation of time slots.
FIGS. 1 and 2 depict flows of signals in which three ONUs are connected. FIG. 1 depicts downlink signals, while FIG. 2 depicts uplink signals. Numbers 1 to 3 with rectangles denote frames of ONU-addressed signals and ONU-departed signals.
FIG. 3 depicts a time relationship of an output request signal (REPORT), an output enable signal (GATE), and an uplink data signal (DATA) exchanged between the ONU and the OLT. FIG. 3 depicts a signal exchanged between one ONU and one OLT. In FIG. 3, t1 and t5 denote transmission time of REPORT, t2 and t4 denote arrival time of GATE, Waiting Time denotes waiting time until signal transmission, and Time Slot denotes a time slot of data transmission.
In many cases, REPORT is transmitted at the end of DATA by piggy back. In that case, t4=t5.
FIG. 4 depicts a time relationship of signals exchanged between three ONUs (ONU1, ONU2, and ONU3) and the OLT. A cycle in which uplink signal transmissions of all ONUs are performed will be called a service cycle. The length of the service cycle is usually not constant and is often dynamically changed according to the output requests from the ONUs.
Ethernet (registered trademark) and PON are standardized by IEEE802.3ah, wherein frame formats of a REPORT message and a GATE message are defined. However, uplink bandwidth allocation methods or algorithms are not defined and are left up to the installation of devices.
Since the uplink signals usually include a plurality of services, traffic is classified, and each class is provided with a priority. For example, three classes, EF (Expedited Forwarding), AF (Assured Forwarding), and BE (Best Effort), are defined in DiffServe (Differentiated Services) standardized by IETF (Internet Engineering Task Force).
EF is a class that guarantees delay and bandwidth, AF is a class that only guarantees bandwidth without guaranteeing delay, and BE is a class that neither guarantees delay nor bandwidth. Their representative applications include VoIP (Voice over IP), file transfer, and normal Internet access, respectively.
An example of a related technology of the bandwidth allocation method includes an algorithm called D1 (see Non-Patent Document 1 (Y. Luo et al., “Bandwidth Allocation for Multiservice Access on EPONs”, IEEE Communications Magazine February 2005, s16-s21)). In the algorithm D1, a maximum value of a service cycle is determined in advance, and fixed bandwidth is allocated to the EF of each ONU within the value. The AF is then allocated based on the REPORTs from the ONUs. At this time, allocation according to the request is performed if the sum of the requests of the AF is less than the remaining bandwidth. BE is allocated according to the request if the bandwidth still remains. The AF is equally allocated to the ONUs if the sum of the requests of the AF exceeds the remaining bandwidth. The BE is not allocated. The calculation and allocation of bandwidth are performed at once after the REPORTs from all ONUs are notified, and GATE is transmitted to each ONU.
FIG. 5 is a block diagram of an allocation module arranged in the OLT of the related technique. IF in FIG. 5 denotes an interface of REPORT and GATE messages exchanged with the ONUs. AM denotes an allocation module, which receives notification of the status of buffer memories of the ONUs from the IF to perform actual bandwidth allocation.
A related technique by the present applicant includes a technique in which a subscriber terminal transmits a warning when the capacity of data to be transmitted is greater than a predetermined threshold and when the capacity is zero, and an optical line terminator changes the maximum number of cells, the average number of cells, and the minimum number of cells allocated to the uplink bandwidth based on the warning to thereby dynamically control the uplink bandwidth (see, for example, Japanese Patent Laid Open Publication No. 2000-358041).
Problems of the conventional techniques will now be described.
The algorithm D1 in Non-Patent Document 1 is relatively concise and can be easily realized in a small-scale PON. However, there is a problem in scalability. More specifically, the control circuit may be subjected to a great burden when the number of ONUs is large, because the allocation is intensively performed after the REPORTs of all ONUs are collected before the start of the next service cycle.
Therefore, costly and fast integrated circuits or CPUs are necessary in the control circuit of a large-scale PON, resulting in an increased cost of the system. Furthermore, the performance may be degraded if a plenty of calculation time is allowed, because the start of the service cycle is delayed, and the bandwidth is wasted.
In the technique in Japanese Patent Laid Open Publication No. 2000-358041, the OLT intensively controls the bandwidth allocation. Therefore, costly and fast integrated circuits or CPUs are necessary in the control circuit of a large-scale PON, and the cost of the system may be increased.